MU-MIMO (multiple-user multi-input multi-output) is a kind of spatial multiplexing technology in physical layer of LTE (Long Term Evolution) and supports multiple users simultaneously in a cell. Current standard releases (Rel. 8-11) already support MU-MIMO, but in a kind of transparent way. An eNode B flexibly performs switching/scheduling between SU-MIMO (single-user MIMO) and MU-MIMO. Furthermore, there is no any enhancement on MU CSI (channel status indicator) calculation since current assumption on CSI calculation is SU-MIMO. The problem of such behavior is that eNode B cannot know accurate CSI information on MU so that MU performance will be degraded due to inaccuracy of MU paring and scheduling.
Such issue is now being investigated in eMIMO session (Further Downlink MIMO Enhancement for LTE-Advanced) in Rel.12 standardization meetings. However, there is no agreement so far. From contributions submitted to the latest several RAN1 meetings (e.g., 3GPP RAN1 #73), there are four types of options as follows:
[Option 1] Introducing a MU-CQI (multiple-user channel quality indicator) offset while a co-scheduled PMI hypothesis is configured by RRC(radio resource control);
[Option 2] Introducing RRC signaling for each CSI process about SU or MU assumption;
[Option 3] Introducing a well defined CSI-IM (CSI interference measurement) measurement interval by a measurement averaging window;
[Option 4] No enhancement for MU (i.e. standard transparent approach by utilizing CSI-IM+rank restriction).
Compared to Options 2-4, Option 1 is supposed to get the best performance as UE can feedback accurate MU CSI information.
The idea of Option 1 (see R1-132484 submitted to 3GPP RAN1 #73, which is incorporated herein by reference) is that UE feedbacks (report) multiple MU-CQI offsets based on co-scheduled PMI (precoding matrix indicator) assumptions. Herein, the co-scheduled PMI is the possible PMI for a possibly co-scheduled UE, which is configured by RRC. The MU-CQI offset is a CQI offset (usually degradation) when introducing a certain co-scheduled PMI relative to the CQI for a desired PMI. The desired PMI is the best PMI that the target UE selects. The co-scheduled PMI assumptions are configured by RRC, as shown in following Table 1.
TABLE 1Co-scheduled PMI assumptions (K = 5) - from R1-132484Desired PMICo-scheduled PMI0{1, 2, 3, 8, 10}1{0, 2, 3, 9, 11}2{0, 1, 3, 8, 10}3{0, 1, 2, 9, 11}4{6, 5, 7, 2, 3}5{7, 4, 6, 0, 3}6{4, 7, 5, 0, 1}7{5, 6, 4, 1, 2}8{0, 2, 9, 10, 11}9{1, 3, 8, 10, 11}10{0, 2, 8, 9, 11}11{1, 3, 8, 9, 10}12{13, 14, 15, 5, 6}13{12, 14, 15, 4, 7}14{12, 13, 15, 4, 7}15{12, 13, 14, 5, 6}
Table 1 assumes the number of co-scheduled PMIs (K) is 5. As seen from Table 1, if the desired PMI index is 0, the co-scheduled PMI index is 1, 2, 3, 8 and 10; if the desired PMI index is 1, the co-scheduled PMI index is 0, 2, 3, 9 and 11, and so on. The MU-CQI offset is calculated corresponding to each co-scheduled PMI assumption. The feedback of the MU-CQI offset relies on a differential CQI table, which needs 2 bits, as shown in Table 2 below.
TABLE 2Differential CQI tableSubband differentialOffset levelCQI value(dB)00112≧23≦−1
In Table 2, the offset level of the MU-CQI offset can be indicated by 2 bits. The value (subband differential CQI value) of the 2 bits is listed in the left column of the table, and the corresponding offset level is listed in the right column of the table. For example, if the value of the 2 bits is 0, that is, the 2 bits are “00”, then the offset level is 0 dB, which means that there is no offset between the CQI with a co-scheduled PMI and the CQI for a desired PMI. If the value of the 2 bits is 3, that is, the 2 bits are “11”, then the offset level is “≦−1 dB”, which means that the CQI with a co-scheduled PMI is smaller than the CQI for a desired PMI by more than 1 dB.
FIG. 1 illustrates an exemplary communication system supporting MU-MIMO for MU-CQI offset feedback, in which there are one eNode B (eNB), one target UE and two possibly co-scheduled UEs. For this system, the target UE reports one CQI for the desired PMI and also reports two CQI offsets 1 and 2 respectively for two possibly co-scheduled PMIs 1 and 2.
The problem of Option 1 above is that the overhead is very large especially if the number of co-scheduled PMIs is large. For example, if the number of co-scheduled PMIs is 5, 10 bits for 5 MU-CQI offsets are needed as each MU-CQI offset needs 2 bits based on the differential CQI table. If the number of co-scheduled PMIs is 10, then 20 bits for 10 MU-CQI offsets are needed.